BMA Concise Guide to Medicine & Drugs


Brand names None

Used in the following combined preparations Rifater, Rifinah, Rimactazid


Drug group Antituberculous drug

Overdose danger rating High

Dependence rating Low

Prescription needed Yes

Available as generic Yes


Isoniazid (also known as INAH and INH) has been in use for over 30 years and remains an effective drug for tuberculosis. It is given alone to prevent tuberculosis and in combination with other drugs for the treatment of the disease. Treatment usually lasts for six months. However, courses lasting nine months or a year may sometimes be prescribed.

Although isoniazid usually causes few adverse effects, one of its side effects is the increased loss of pyridoxine (vitamin B6) from the body. This effect, which is more likely with high doses, is rare in children but common among people with poor nutrition. Since pyridoxine deficiency can lead to irreversible nerve damage, supplements are usually given.


Your drug prescription is tailored for you. Do not alter dosage without checking with your doctor.

How taken/used Tablets, liquid, injection.

Frequency and timing of doses Normally once daily.

Dosage range Adults 300mg daily. Children According to age and weight.

Onset of effect Over 2–3 days.

Duration of action Up to 24 hours.

Diet advice Take 30 minutes before food because food decreases absorption of isoniazid.

Storage Keep in original container at room temperature out of the reach of children. Protect from light.

Missed dose Take as soon as you remember. If your next dose is scheduled within 8 hours, take a single dose now and skip the next.

Stopping the drug Take the full course. Even if you feel better the infection may still be present and may recur if treatment is stopped too soon.


Seek immediate medical advice in all cases. Take emergency action if breathing difficulties, seizures, or loss of consciousness occur.


Serious problems are uncommon, but all adverse effects should receive prompt medical attention because of the possibility of nerve or liver damage; such damage is more likely with long-term use (see Prolonged use). Adverse effects include nausea, vomiting, fatigue, weakness, numbness, tingling, rash, and mood changes. If you develop blurred vision, jaundice, twitching, or muscle weakness, stop taking the drug and consult your doctor without delay.


Alcohol and rifampicin Large quantities of alcohol may reduce the effectiveness of isoniazid. If the two are taken together, the likelihood of liver damage is increased; if, in addition, rifampicin is being taken, the risk of liver damage is increased further.

Theophylline Isoniazid may increase levels and effects of theophylline.

Anti-epileptics The effects of these drugs may be increased with isoniazid.

Antacids These drugs may reduce the absorption of isoniazid.

Ketoconazole Isoniazid reduces the blood concentration of ketoconazole.


Be sure to tell your doctor if:

· You have long-term liver or kidney problems.

· You have had liver damage following isoniazid treatment in the past.

· You have problems with drug or alcohol abuse.

· You have diabetes.

· You have porphyria.

· You have HIV infection.

· You have had epileptic seizures.

· You are taking other medicines.

Pregnancy No evidence of risk. Discuss with your doctor.

Breast-feeding The drug passes into the breast milk and may affect the baby. The infant should be monitored for signs of toxic effects. Discuss with your doctor.

Infants and children Reduced dose necessary.

Over 60 Increased likelihood of adverse effects.

Driving and hazardous work No special problems.

Alcohol Avoid excessive amounts.


Pyridoxine (vitamin B6) deficiency may occur with prolonged use and lead to nerve damage. Supplements are usually prescribed. There is also a risk of serious liver damage.

Monitoring Periodic blood tests are usually performed to monitor liver function.